1. Field of the Invention
The present invention relates to a lithography process. More particularly, the present invention relates to an immersion lithography process and a mask layer structure applied in the same.
2. Description of the Related Art
Lithography is one of the most important techniques utilized in semiconductor manufacture, and is used in any process required to define patterns, such as a wafer layer patterning process or a doped-region defining process. A lithography process generally includes an exposure step and a development step, wherein the exposure step utilizes a light source to irradiate a photoresist layer directly or through a photomask to induce chemical reactions in exposed portions. The development step is conducted to remove the exposed portion and form photoresist patterns, thus completing the transfer of photomask patterns or virtual patterns.
Among various lithography techniques, the immersion lithography technique has been well developed to cope with the miniaturization of IC devices accompanying with the increase in degree of IC integration. An immersion lithography process is conducted in a liquid-phase environment, and has a higher resolution because the refractive index of a liquid is closer to that of the photoresist material as compared with air. Therefore, the dimensions of IC devices can be reduced with the immersion lithography technique.
However, since the photoresist layer directly contacts with the immersion liquid during the exposure step of an immersion lithography process, the immersion liquid is easily contaminated by the chemicals out-diffusing from the photoresist layer. The immersion liquid may also diffuse into the photoresist layer and alter the properties of the latter. Therefore, the critical dimension and the uniformity of the photoresist patterns are adversely affected.